The observation that women who work at night have an increased breast cancer risk is intriguing, but the biological mechanisms explaining this association are not yet established. One theory is that the hormone melatonin, which is produced at higher levels during periods of darkness, is involved. There is considerable experimental evidence to support the anti-cancer activities of melatonin. However, results from the limited observational cohort studies that have examined the relation between urinary levels of melatonin and breast cancer risk have been inconsistent, and additional studies are needed to evaluate this promising, but as yet unconfirmed, association. We propose to conduct a nested case-control study of 284 invasive breast cancer cases and 568 control subjects within the WHI Observational Study (OS) to investigate if higher urinary levels of the melatonin metabolite 6-sulfatoxymelatonin are associated with a decreased risk of postmenopausal breast cancer. This study is important to conduct in WHI because it is one of very few prospective cohorts that have urinary specimens collected appropriately to evaluate this hypothesis. This nested case-control study will afford the opportunity to explore the association between levels of urinary melatonin and breast cancer risk by the time interval between urinary specimen collection and breast cancer development. This study is important to clarify the role of melatonin in breast cancer etiology, especially because the knowledge gained has the potential to lead to important new strategies to prevent breast cancer. 1 PUBLIC HEALTH RELEVANCE: Establishing melatonin as a risk factor for breast cancer may lead to important new intervention strategies to prevent breast cancer.